This is really a hard topic to give a recommendation on as what is firm to one person isn't firm to the next and so on.
We like firm/firm and have been real happy with the foam that we got from the Foam Factory...
http://www.thefoamfactory.com/
...they have a wide selection of firmness and quality and good service along with good prices and for us the free shipping really pays off.
We bought the Lux Foam - High Quality which is the firmest and has the longest live span. We used it in the Mac for a couple months on the water and slept so well that we bought the same stuff for a mattress in the tear drop trailer, a mattress for the back of the Suburban and then even got it for our bed at home and are also going to use it in the Endeavour. Can you tell we like it

. So now about any place we go we are sleeping on the same mattress. I'm not saying it is for everyone, but if you like firm you will like it. They also have others that aren't as firm.
We have been using the 3 inch and since it is firm we don't go through it and it gives us more headroom in the boat. We use the same at home with a sheet of plywood under it and over the beds inter-springs, but I'm just going to make a platform bed so that we can have storage under it. If you went with one that wasn't as firm then 4-5 inch might be better.
We also used the same foam for our cabin ....
http://purplesagetradingpost.com/sumner/macgregor-canvas/canvas-20.html
...cushions. We might try the 4 inch for cabin cushions in the Endeavour or 3 inches with 1 inch of dryfast on the bottom. If I'm on those cushions 4 hours or so my boney butt starts to go through

. For a couple hours the 3 inch are fine and Ruth has no problems with the 3 inch at all.
We used their dryfast for our cockpit cushions...
http://purplesagetradingpost.com/sumner/macgregor2/outside-31-a.html
... and love it. Along with an open fabric that breaths from Sailrite no more soggy cockpit cushions. We ordered cut foam from the link above and then I found out how easy it is to cut with a $15 electric carving knife and now we order from their wholesale side......
http://www.foamdistributing.com/
better prices yet,
Sum
[FONT=Arial, sans-serif]
Our Endeavour 37[/FONT]
[FONT=Arial, sans-serif]
Our Trips to Utah, Idaho, Canada, Florida[/FONT]
[FONT=Arial, sans-serif]
Our MacGregor S Pages[/FONT]
[FONT=Arial, sans-serif]
Mac-Venture Links[/FONT]